This invention relates to a family of compounds which can be crosslinked by photon irradiation and to a process for using these compounds. The photons used for irradiation are photons issuing from either visible or invisible radiation, particularly ultraviolet radiation, X-radiation or gamma radiation.
The phenomenon of crosslinking of so-called negative resins is well known. Resins such as these harden under the effect of photon irradiation by the establishment of "bridging" bonds between the various polymer chains which they contain. These bonds result in the formation of a three-dimensional network. From the point of view of the intrinsic properties of the resins, this is reflected in an increase in their molecular weight which is responsible for their insolubility in developer solvents.
Commercially available crosslinkable resins are formed:
either by a polymer containing both a crosslinkable group and a photosensitive group: PA1 or by a "system" containing both a polymer having a crosslinkable entity and a photoinitiator or photosensitiser compound. PA1 4 mJ/cm.sup.2 (in photon irradiation at a wavelength of 404.7 nm) for the resin KPR (a Kodak product); PA1 1.3 mJ/cm.sup.2 (in photon irradiation at wavelengths of 351.1 and 363.8 nm) for the epoxy resin commercially available under the name "PSE-2"; a sensitivity of this order requires an exposure time of around 10 seconds under normal working conditions in a factory. PA1 the substituent .DELTA. denotes a radical or a plurality of radicals such as: PA1 X denotes a halogen atom, such as F or Cl; PA1 T.sub.1 and T.sub.2 denote aromatic radicals (which may be identical) containing from 4 to 20 carbon atoms (phenyl, thienyl, furanyl, pyrazolyl); PA1 Y denotes a radical of the following group: ##STR8## R.sub.3 --N (with R.sub.3 =H or an alkyl or acyl radical or a C--C bond); ##STR9## (where R.sub.1 and R.sub.2 represent identical or different radicals such as H or a C.sub.1-4 -alkyl radical or a C.sub.2-4 -alkenyl radical).
The sensitivity of two known crosslinkable compositions is given in the following by way of order of magnitude:
By virtue of the present invention, it is possible to obtain excellent sensitivity levels, i.e. in favourable cases of the order of one tenth of a millijoule per square centimeter.
In addition, the compounds according to the invention have other advantages which will be discussed hereinafter.
According to the invention, there is provided a family of compounds crosslinkable by photon irradiation comprising at least one copolymer of which the chemical formula comprises a thiirane ring: ##STR2## wherein said polymer contains at least one monomer formed by 2,3-epithiopropyl methacrylate corresponding to the developed formula: ##STR3## wherein R is selected from the group consisting of an alkyl group of 1 to 4 carbon atoms and hydrogen.
According to one aspect of the invention, the above monomer is copolymerized with a vinyl monomer corresponding to the formula: ##STR4## in which R' is either a radical H or an alkyl group C.sub.n H.sub.2n +1 where n is an integer from 1 to 10, R" being an alkyl group containing from 1 to 5 carbon atoms.
According to another aspect of the invention, said family additionally comprises at least one of the following two salts:
an aryl diazonium salt corresponding to the general formula: ##STR5## and an aryl iodonium salt corresponding to the formula: ##STR6## where n=0 or 1.
In these formulae:
--OH; --NH.sub.2 ; --CHO; --NO.sub.2 ; --OCH.sub.3 ; ##STR7## the letters a and b denote integers from 1 to 5; the element M is a metal such as Fe, Sn, Sb or Bi or another element such as B, P, As;
The copolymer of 2,3-epithiopropyl methacrylate and the vinyl monomer may be obtained as follows:
Glycidyl methacrylate corresponding to the formula: ##STR10## is reacted with thiourea corresponding to the formula: ##STR11## at ambient temperature in a mixture of water and ethanol. The reaction gives 2,3-epithiopropyl methacrylate corresponding to the formula: ##STR12##
This compound is extracted with ether. The solution is dried with sodium sulphate. The ether is removed in vacuo and the remaining product is distilled in vacuo.
The epithiopropyl methacrylate thus obtained is mixed with a quantity of vinyl monomer in a proportion of from 20 to 60% by weight thereof in a solvent, such as benzene. Azobis-isobutyronitrile, which acts as copolymerisation catalyst, is then added to the resulting mixture. The mixture is heated under nitrogen at 80.degree. C. over a period ranging from one to several hours. After cooling, the copolymer is obtained by precipitation with methanol, followed by drying in vacuo.
For example, the vinyl monomer may be butyl methacrylate or ethylacrylate.
The invention will be better understood from the following description and examples which in particular define a process for using the resin.
In the formula of the diazonium salt, the ion: EQU (M X.sub.b+a).sup.a-
is a complex halogenated anion (X=F or Cl) with a metallic or non-metallic element M (such as defined above) of which the charge is a. The number of negative charges also represents the number of halogenated ions complexed with the Lewis acid M X.sub.b.
A similar situation with a single negative charge on the halogenated anion prevails in the case of the diaryl iodonium salt.
By photolysing, these salts produce the Lewis acids M X.sub.b which chemically initiate the crosslinking process by a cationic mechanism. This mechanism consists in the formation of a covalent bond between the sulphur atom of the thiirane cycles, which gives an electron pair, and the element M which accepts this electron pair.
The formation of a complex having an ionic tendency, which results from the more intensive polarisation of one of the C-S bonds of the thiirane rings, enables the opening of other thiirane rings to be initiated, giving rise to a cationic crosslinking reaction: ##STR13##
The phase (3) is the initiation phase; the phase (4) is the propagation phase.
Since the thiirane ring is not (or only slightly) affected by agents forming free radicals, but is highly sensitive to ion attacks, the radiation dose required for crosslinking the "thiirane" resins in the presence of Lewis acids is lower than when they are irradiated on their own.
The photocrosslinking of the compounds according to the invention is a reaction of the initiated type because, once the degradation of the photoinitiators (aryl diazonium or aryl iodonium salts) has been initiated, i.e. after formation of the Lewis acids, irradiation may be stopped and the crosslinking reaction may continue in the absence of irradiation like any polymerisation reaction initiated by Lewis acids.
Although the resins according to the invention used on their own can only be sensitised in a narrow spectral region, the addition of the above-mentioned photoinitiators not only enables sensitivity to be increased, it also enables the spectral sensitivity range to be widened by extending it in some cases into the range of visible light.
Thus, by acting on the substituent of the aryl group in the case of the aryl diazonium salts, the absorption range is displaced towards the visible. For example, with a substituent such as a halogen atom or an alkyl radical, a slight displacement is obtained towards the major wavelengths (up to 273 nm for the substituent Cl). This displacement becomes very considerable when groups containing non-binding electrons, such as
--OH; --NH.sub.2 ; --CHO; --NO.sub.2 ; --OCH.sub.3 ##STR14## are substituted.
In order to obtain sensitisation in the region of visible light, it is necessary for example to add to the above-mentioned products so-called "photo-optical" products which serve to sensitise the photoinitiators (for example the aryl diazonium and aryl iodonium salts). In general, sensitisation is obtained by the transfer of energy from the "triplet" state of the sensitiser (donor) to the "triplet" state of the initiator (acceptor), photodegradation taking place under the effect of this input of energy.
Depending upon the required spectral region of sensitisation, the "photo-optical" products may belong to various families of dyes, provided that these dyes do not destroy the Lewis acids formed during the initiation phase and that their energy levels are adapted to those of the photoinitiators used so as to provide for an effective transfer of energy.
The dyes in question include, for example, coumarins, xanthenes, acridines, thiopyronines, safranines, thiazines, oxazines, cyanines (carbocyanines, oxacyanines, thiacyanines), coloured polycyclic aromatic hydrocarbons and, finally, compounds containing para-substituted aminostyryl groups.
As mentioned above, the compounds according to the invention may be crosslinked by ionising radiation, such as X-radiation or gamma radiation. Their sensitivities to radiation of this type are approximately ten times higher than those of the corresponding epoxy resins, for example 80 J/cm.sup.3 as opposed to 740 J/cm.sup.3 for an irradiation wavelength of 8.34 Angstroms.
In the case of irradiation by soft X-rays (spectral region where the photo-electrical absorption is preponderant over the Compton diffusion), for example for photons with energy levels of from about 0.1 to 10 KeV, it is possible to increase the sensitivity of the compounds according to the invention to excess in relation to that of epoxy resins by increasing their coefficient of absorption to the X-rays by optimisation of the irradiation wavelength, as explained hereinafter.
It is known that the sensitivity of a resin to radiation depends upon the energy of the incident photons which it absorbs. Its so-called "X-photoelectric" absorption coefficient is a weighted sum of the absorption coefficients of the various atomic species by which it is formed. These coefficients increase with the incident wavelength (proportionally to the third power) and with the atomic number of the absorbent (proportionally to the fourth power) between the discontinuities which characterise the photoelectric absorption. These discontinuities which correspond to the ionisation potentials of the various energy levels are characteristic of each type of atom.
In order to increase the sensitivity of the compounds according to the invention to soft X-rays, the wavelength of the X-rays is selected for example in the spectral region situated beyond a discontinuity of oxygen and within one of the following discontinuities of sulphur.
This is the case between the wavelengths of 76.05 Angstroms (discontinuity L.sub.II of sulphur) and 23.30 Angstroms (discontinuity K of oxygen) where the absorption coefficient of sulphur is respectively from 7 to 20 times greater than that of oxygen, although these two coefficients have substantially the same value for shorter wavelengths, i.e. 23,000 cm.sup.-1 at 22.3 Angstroms; the absorption coefficient of oxygen becoming greater than that of sulphur at longer wavelengths (i.e. respectively 55 700 and 24 900 cm.sup.-1 at 109 Angstroms).
The following table gives the absorption coefficients of the oxygen and sulphur atoms for certain wavelengths (or photon energy levels) situated in the range favourable to an increase in sensitivity of the compounds according to the invention, the wavelengths selected being those of the lines of various atoms emitting in the region in question.
______________________________________ Nature of the line .sup..mu. S/.mu. .lambda.(A) E(KeV) emitted .sup..mu. O(cm.sup.-1) .sup..mu. S(cm.sup.-1) O ______________________________________ 23.6 0.525 O-K.sub..alpha. 1368 26931 19.7 27.4 0.452 Ti-L.sub..alpha.1.2 2002 37943 18.95 31.4 0.395 Ti-L.sub.1 2825 50798 18.0 31.6 0.392 N-K.sub..alpha. 2881 51605 17.9 44.7 0.277 C-K.sub..alpha. 6890 99236 14.4 58.4 0.212 W-N.sub.V,II 13247 126601 9.6 64.4 0.193 MO-M.sub..zeta. 16587 144465 8.7 67.6 0.183 B-K.sub..alpha. 18844 153553 8.15 72.2 0.172 Nb-M.sub..zeta. 21842 165145 7.6 ______________________________________
In the above-mentioned region, the use of photoinitiators such as the complex aryl diazonium and diaryl iodonium salts enables the sensitivity of the compounds containing a thiirane ring to be increased, the X or gamma photons initiating the degradation of these salts into Lewis acids in the same way.
In addition, in the region of the soft X-rays where the photoelectric absorption is preponderant, the choice of the photoinitiator, particularly the choice of the element M in the general formula of the complex salt, enables the sensitivity of the compound according to the invention to be increased, as shown hereinafter.
It is known that, in general, the interaction mechanism between radiation and sensitive resin uses a transfer of energy from the chemical group or the absorbent atom of the resin towards its sensitive part. The effectiveness of this transfer is dependent upon the distance between these two "entities".
Accordingly, it is possible to use to advantage the narrow distance between the absorbing group and the sensitive group in case where the above mentioned complex salts are used, i.e. the proximity between the sulphur atom of the thiirane ring and the element M of the Lewis acid. Accordingly, this element is selected according to its X-photon absorption coefficient.
The following table shows the absorption coefficients in cm.sup.-1 for five elements M in five cases of use characterised by the emission of different atomic lines.
______________________________________ .lambda. 8.34 A 13.3 A 23.6 A Element line K.sub..alpha.1.2 of Al line L.sub..alpha.1.2 of Cu line K.sub..alpha. of ______________________________________ O Fe 27071 84173 31504 Sn 24350 70344 168788 Sb 23522 63625 22870 Bi 26580 62137 123689 As 30942 14359 50453 ______________________________________ .lambda. 44.7 A 72.2 A Element line K.sub..alpha. of C line M.sub..zeta. of Nb ______________________________________ Fe 104724 221496 Sn 46287 47932 Sb 44167 44181 Bi 92918 32019 As 159867 268336 ______________________________________